Hyphenated FT-IR-attenuated total reflection and electrochemical impedance spectroscopy technique to study the water uptake and potential stability of polymeric solid-contact ion-selective electrodes

A new hyphenated method utilizing FT-IR-attenuated total reflection (ATR) and electrochemical impedance spectroscopy (EIS) is presented to correlate the water uptake with concomitant potential and impedance changes of polymeric coated-wire electrodes (CWEs) and solid-contact ion-selective electrodes (SCISEs). The Ca(2+)-selective silicone rubber (RTV 3140) based SCISEs with poly(3-octylthiophene) (POT) as the solid-contact (SC) showed good correlation between a very low water content at the Pt-coated ZnSe substrate/SC interface and a superior potential stability. This is due to the hydrophobicity of both RTV 3140 and POT and the approximately 2 orders of magnitude lower water diffusion coefficients in POT compared to RTV 3140. Practically no potential drift could be observed during 24 h when unconditioned CaSCISEs were contacted with 10(-3) M CaCl(2), in contrast to the Ca(2+)-selective CWEs with considerably higher water uptake and potential drift. The CaSCISEs had a fast Nernstian response with a detection limit of 8 × 10(-9) M Ca(2+) and a good reproducibility and stability of the standard potential, which indicates that the CaSCISEs does not require any conditioning prior to use.     

Polyaniline nanoparticle-based solid-contact silicone rubber ion-selective electrodes for ultratrace measurements

Silicone rubber (SR)-based solid-contact ion-selective electrodes (ISEs) have been prepared for the first time with an electrically conducting polymer as the solid-contact (SC) layer. The Ca(2+)- and Ag(+)-selective electrodes were based on the ionophores ETH 1001 and o-xylylenebis(N,N-diisobutyl dithiocarbamate), respectively, integrated in room temperature vulcanizing silicone rubber (RTV 3140). The SC consisted of a polyaniline nanoparticle dispersion, which was found to considerably lower the impedance of the SCISEs in comparison to the SR-based coated wire electrodes (CWE). For the CaSCISEs, the bulk membrane resistance decreased from 700 MΩ (CaCWE) to 35 MΩ. Both the Ca(2+)- and Ag(+)-selective SCISEs exhibited nanomolar detection limits with fast Nernstian responses down to 10(-8) M. The potential response of the SCISEs was not influenced by light. The selectivities of the CaSCISEs were similar and for the AgSCISE better than their plasticized PVC-based analogues. Thus, SR seems to be a viable alternative to PVC membranes in ISE applications that require low water uptake, good adhesion, and robust and fast potential responses at submicromolar sample concentrations.     

Pulsed galvanostatic control of solid-state polymeric ion-selective electrodes

We report on galvanostatically controlled solid-state reversible ion-selective sensors for cationic analytes utilizing a conducting polymer as a transduction layer between the polymeric membrane and electron-conductive substrate. The instrumental control of polymeric membrane ion-selective electrodes based on electrochemically induced periodic ion extraction in alternating galvanostatic/potentiostatic mode was introduced recently creating exciting possibilities to detect clinically relevant polyions such as heparin and protamine and drastically improve the sensitivity of ion-selective sensors limited by the Nernst equation. The present study forms the basis for development of reliable, robust, and possibly maintenance-free sensors that can be fabricated using screen-printing technology. Various aspects of the development of solid-contact galvanostatically controlled ion-selective electrodes with a conducting polymer as a transduction layer are considered in the present work on the example of a model system based on a sodium-selective membrane. The protamine-selective solid-contact sensor was fabricated and characterized, which represents the next step toward commercially viable polyion sensing technology. A substantial improvement of a low detection limit (0.03 mg L-1) was achieved. A simplified diffusion-based theoretical model is discussed predicting the polarization at the interface of the conducting polymer and the membrane, which can cause the disruption of the sensor response function at relatively small current densities.     

Electrochemical sample matrix elimination for trace-level potentiometric detection with polymeric membrane ion-selective electrodes

Potentiometric sensors are today sufficiently well understood and optimized to reach ultratrace level (subnanomolar) detection limits for numerous ions. In many cases of practical relevance, however, a high electrolyte background hampers the attainable detection limits. A particularly difficult sample matrix for potentiometric detection is seawater, where the high saline concentration forms a major interfering background and reduces the activity of most trace metals by complexation. This paper describes for the first time a hyphenated system for the online electrochemically modulated preconcentration and matrix elimination of trace metals, combined with a downstream potentiometric detection with solid contact polymeric membrane ion-selective microelectrodes. Following the preconcentration at the bismuth-coated electrode, the deposited metals are oxidized and released to a medium favorable to potentiometric detection, in this case calcium nitrate. Matrix interferences arising from the saline sample medium are thus circumvented. This concept is successfully evaluated with cadmium as a model trace element and offers potentiometric detection down to low parts per billion levels in samples containing 0.5 M NaCl background electrolyte.     

Dynamic diffusion model for tracing the real-time potential response of polymeric membrane ion-selective electrodes

A numerical solution for the prediction of the time-dependent potential response of a polymeric-based ion-selective electrode (ISE) is presented. The model addresses short- and middle-term potential drifts that are dependent on changes in concentration gradients in the aqueous sample and organic membrane phase. This work has important implications for the understanding of the real-time response behavior of potentiometric sensors with low detection limits and with nonclassical super-Nernstian response slopes. As a model system, the initial exposure of membranes containing the well-examined silver ionophore O,O' '-bis[2-(methylthio)ethyl]-tert-butylcalix[4]arene was monitored, and the large observed potential drifts were compared to theoretical predictions. The model is based on an approximate solution of the diffusion equation for both aqueous and organic diffusion layers using a numerical scheme (finite difference in time and finite elements in space). The model may be evaluated on the basis of experimentally available parameters and gives time-dependent information previously inaccessible with a simpler steady-state diffusion model. For the cases studied, the model gave a very good correlation with experimental data, albeit with lower than expected diffusion coefficients for the organic phase. This model may address numerous open questions regarding the response time and memory effects of low-detection-limit ion-selective electrodes and for other membrane electrodes where ion fluxes are relevant.     

Lifetime of ion-selective electrodes based on charged ionophores

The lifetime of solvent polymeric ion-selective electrodes (ISEs) is limited by leaching of the membrane components into the sample solutions. In this article, leaching of charged ionophores is discussed. Because of the electroneutrality principle, the loss of the charged ionophore into the sample must be accompanied by parallel transport of an ion of the opposite charge sign into the sample or by ion exchange with a sample ion of the same charge sign. Because ionic sites of high lipophilicity are available, the loss of ionic sites is, in general, not a concern. Therefore, it is assumed here that the cotransported or ion-exchanging ions are primary or interfering ions forming complexes with the ionophore. A general theory that allows quantification of ionophore lipophilicities and a discussion of changes in the membrane composition and selectivity with time is presented. A high complex stability and high analyte concentrations diminish the rate of ionophore loss into the sample if a charged ionophore is coextracted from the membrane into the sample together with an analyte ion of opposite charge. On the other hand, if the charged ionophore has the same charge sign as the ion that it binds, a large binding constant and high analyte concentrations enhance ionophore leaching into the sample. The model is applied to interpret results for an electrically charged ionophore, for which selectivity changes as a function of the leaching time were observed and the lipophilicity was determined with potentiometric measurements. Using the lipophilicities of neutral ionophores, as described previously, and the lipophilicities of charged ionophores, as described here, a direct comparison of the expected leaching rates of charged and neutral ionophores has become possible.     

Improved detection limits and unbiased selectivity coefficients obtained by using ion-exchange resins in the inner reference solution of ion-selective polymeric membrane electrodes

By using a high concentration of an interfering ion and a low one of the primary ion in the inner reference solution of polymeric membrane ion-selective electrodes (ISEs), the lower detection limit may be improved and unbiased thermodynamic selectivity coefficients may be obtained. To this purpose, a cation-exchange resin is used here to keep the low concentration of the primary cation constant. Different compositions of the internal solution are required for obtaining optimal lower detection limits and unbiased selectivity coefficients. All ISEs studied here, i.e., for K+, Ca2+, and NH4+, based on valinomycin, ETH 5234, and nonactin/monactin, respectively, show improved lower detection limits in the range of 10(-7.6) (NH4+) to 10(-8.8) M (Ca2+). Nernstian responses and, therefore, unbiased selectivity coefficients are obtained with the K+-ISE for the discriminated ions, Na+, Mg2+, and Ca2+.     

Continuous flow analytical microsystems based on low-temperature co-fired ceramic technology. Integrated potentiometric detection based on solvent polymeric ion-selective electrodes

In this paper, the low-temperature co-fired ceramics (LTCC) technology, which has been commonly used for electronic applications, is presented as a useful alternative to construct continuous flow analytical microsystems. This technology enables not only the fabrication of complex three-dimensional structures rapidly and at a realistic cost but also the integration of the elements needed to carry out a whole analytical process, such as pretreatment steps, mixers, and detection systems. In this work, a simple and general procedure for the integration of ion-selective electrodes based on liquid ion exchanger is proposed and illustrated by using ammonium- and nitrate-selective membranes. Additionally, a screen-printed reference electrode was easily incorporated into the microfluidic LTCC structure allowing a complete on-chip integration of the potentiometric detection. Analytical features of the proposed systems are presented.     

An experimental study of membrane materials and inner contacting layers for ion-selective K+ electrodes with a stable response and good dynamic range

The goal was to identify formulations for use in valinomycin K(+) ion-selective electrodes that could routinely achieve a detection limit of <10(-6) M, even after repeated use and exposure at higher K+ activity (0.1 M) and without the requirement for special pretreatment or conditioning in low K+ activity (10(-3) M). Electrodes that would be characterized by high potential stability were sought in this work. Valinomycin-containing membranes with diffusion coefficient of approximately 10(-11) cm(2) s(-1), formulated from methacrylic/acrylic polymers with or without plasticizer, were compared with plasticized PVC membranes (diffusion coefficient 10(-8) cm(2) s(-1)). The methacrylic/acrylic membranes without plasticizer were shown to give an order of magnitude lower detection limit, when compared with PVC-dioctyl sebacate and o-nitrophenyl octyl ether plasticized methacrylic/acrylic polymers under the same conditions, highlighting the influence of plasticizer on the detection limit. As predicted from current theoretical derivation, the inner contacting layer in the ion-selective electrode construction was shown to be highly influential in maintaining the detection limit below 10(-6) M with use and with poly(pyrrole) providing the inner contact ion-to-electron transduction function, a further order of magnitude improvement in the lower detection limit could be maintained for both chloride and hexacyanoferrate doped poly(pyrrole), when 2% ionophore was employed in the ion-selective membrane. This formulation showed extraordinary stability and reproducibility in terms of measurement range and drift over extended measurement testing, with close to Nernstian slopes. At higher ionophore concentrations (4%), the apparent selectivity of the electrode was improved at the expense of detection limit and the nature of the poly(pyrrole) dopant ion became important in determining the dominant exchange processes at the poly(pyrrole)/ion-selective membrane interface.     

Rotating disk potentiometry for inner solution optimization of low-detection-limit ion-selective electrodes

The extent of optimization of the lower detection limit of ion-selective electrodes (ISEs) can be assessed with an elegant new method. At the detection limit (i.e., in the absence of primary ions in the sample), one can observe a reproducible change in the membrane potential upon alteration of the aqueous diffusion layer thickness. This stir effect is predicted to depend on the composition of the inner solution, which is known to influence the lower detection limit of the potentiometric sensor dramatically. For an optimized electrode, the stir effect is calculated to be exactly one-half the value of the case when substantial coextraction occurs at the inner membrane side. In contrast, there is no stir effect when substantial ion exchange occurs at the inner membrane side. Consequently, this experimental method can be used to determine how well the inner filling solution has been optimized. A rotating disk electrode was used in this study because it provides adequate control of the aqueous diffusion layer thickness. Various ion-selective membranes with a variety of inner solutions that gave different calculated concentrations of the complex at the inner membrane side were studied to evaluate this principle. They contained the well-examined silver ionophore O,O' '-bis[2-(methylthio)ethyl]-tert-butylcalix[4]arene, the potassium ionophore valinomycin, or the iodide carrier [9]mercuracarborand-3. Stir effects were determined in different background solutions and compared to theoretical expectations. Correlations were good, and the results encourage the use of such stir-effect measurements to optimize ISE compositions for real-world applications. The technique was also found to be useful in estimating the level of primary ion impurities in the sample. For an iodide-selective electrode measured in phosphoric acid, for example, apparent iodide impurity levels were calculated as 5 x 10(-10) M.     

Effect of lipophilic ion-exchanger leaching on the detection limit of carrier-based ion-selective electrodes

The equilibrium partitioning of lipophilic ion-exchanger salts from ion-selective polymeric membrane electrodes (ISEs) and its possible effect on the lower detection limit of these sensors is described. Predictions are made on the basis of various parameters, including the knowledge of tetraphenylborate potassium salt partitioning constants, the selectivity of ionophore-free ion-exchanger membranes, and ionophore stability constants in the membrane. Ion-exchanger lipophilicities are significantly increased if the membrane contains an ionophore that strongly binds the primary ion. Predicted detection limits are on the order of 10(-5)-10(-8) M for ionophore-free membranes, and may reach levels as low as 10(-18) M with adequate ionophores in the membrane. Experiments are performed for well-described lead-selective membranes containing different tetraphenylborate derivatives, and detection limits appear to be independent of the ion-exchanger used. However, they are much higher if a more hydrophilic carborane cation-exchanger is incorporated in the membrane. The first finding confirms recent theory, which states that transmembrane ion fluxes, given by a small level of ion-exchange at the sample side by interfering ions, normally dictate the detection limit of these sensing systems. Predicted detection limits on the basis of ion-exchanger leaching alone are here listed for a number of analytically relevant cases. For potassium-selective electrodes containing BME-44 and tetraphenylborate as ion-exchanger, the experimental detection limits are in agreement with predicted values. These results suggest that the detection limit of many current ISEs for ultratrace level analysis are, in optimal cases, dictated by transmembrane ion fluxes; however, because improved chemical solutions are being developed to reduce such effects, simple ion-exchanger partitioning may indeed become an important mechanism that can give higher detection limits than practically desired, and should not be ruled out.     

Direct sensing of total acidity by chronopotentiometric flash titrations at polymer membrane ion-selective electrodes

Polymer membrane ion-selective electrodes containing lipophilic ionophores are traditionally interrogated by zero current potentiometry, which, ideally, gives information on the sample activity of ionic species. It is shown here that a discrete cathodic current pulse across an H (+)-selective polymeric membrane doped with the ionophore ETH 5294 may be used for the chronopotentiometric detection of pH in well-buffered samples. However, a reduction in the buffer capacity leads to large deviations from the expected Nernstian response slope. This is explained by the local depletion of hydrogen ions at the sample-membrane interface as a result of the galvanostatically imposed ion flux in direction of the membrane. This depletion is found to be a function of the total acidity of the sample and can be directly monitored chronopotentiometrically in a flash titration experiment. The subsequent application of a baseline potential pulse reverses the extraction process of the current pulse, allowing one to interrogate the sample with minimal perturbation. In one protocol, total acidity is found to be proportional to the magnitude of applied current at the flash titration end point. More conveniently, the square root of the flash titration end point time observed at a fixed applied current is a linear function of the total acid concentration. This suggests that it is possible to perform rapid localized pH titrations at ion-selective electrodes without the need for volumetric titrimetry. The technique is explored here for acetic acid, MES and citric acid with promising results. Polymeric membrane electrodes based on poly(vinyl chloride) plasticized with o-nitrophenyl octyl ether in a 1:2 mass ratio may be used for the detection of acids of up to ca. 1 mM concentration, with flash titration times on the order of a few seconds. Possible limitations of the technique are discussed, including variations of the acid diffusion coefficients and influence of electrical migration.     

Observation of high gain in a liquid-crystal panel with photoconducting polymeric layers

A novel, to our knowledge, liquid-crystal panel suitable for real-time holographic purposes has been prepared. A nematic liquid-crystal layer sandwiched between photoconducting polymeric layers, when exposed to a sinusoidal light-intensity pattern, shows efficient formation of refractive-index gratings. The unique feature of the presented panel is its ability to switch energy from beam to beam in a manner similar to the charge-diffusion-controlled photorefractive effect. In a two-wave-mixing experiment multiple orders of diffraction are present, and a very high two-beam coupling-gain ratio (2.5) and a net exponential gain coefficient of ? = 931 cm(-1) have been measured. This gain was achieved in samples biased by a dc external electric field and tilted with respect to the beam-incidence bisector at 45 degrees . The time constants for grating formation and erasure in the studied system are functions of the applied voltage and can be made as short as a few milliseconds under favorable conditions. The mechanism of beam coupling is linked with an electric-field-driven reorientation of the nematic director as a result of a spatially modulated space-charge field created by light in a photoconducting poly(3-octyl)thiophene polymeric layer.